Lightning and the Hazards It Produces for Explosive Facilities - ACS

4 Lightning

a n d t h e H a z a r d s It P r o d u c e s f o r E x p l o s i v e

Facilities

Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 23, 2018 | https://pubs.acs.org Publication Date: April 6, 1979 | doi: 10.1021/bk-1979-0096.ch004

RODNEY B. BENT Atlantic Scientific Corp., P.O. Box 3201, Indialantic, FL 32903

Lightning is a natural phenomenon which poses a potential hazard to people, structures, and equipment unless adequate protection is provided. The type of protection required is r e lated to the nature and function of the facility. The decision making process involves a number of interrelated factors which should be considered when determining the need for protection. A knowledge of the basic lightning process can lead to a much better understanding of these lightning protection techniques and the resulting level of protection. The design of satisfactory lightning protection systems can, therefore, only be achieved with a knowledge of the mechanism and characteristics of a lightning strike and the related problems that a steep voltage wavefront has on inadequate bonding and grounding. Lightning induced line surges can also cause major damage to electrical or electronic systems. A considerable proportion of the damage caused by such surges can be eliminated with careful planning of protection equipment. These line surges can also cause extra bits in computer software, which may lead to false decision making by the computer. The

Lightning P r o c e s s in a Cloud-to-Ground A

Discharge

c l o u d - t o - g r o u n d l i g h t n i n g d i s c h a r g e i s m a d e u p of o n e o r

m o r e intermittent partial discharges.

T h e total d i s c h a r g e ,

w h o s e t i m e d u r a t i o n i s o f t h e o r d e r of 0. 5 s e c o n d s , flash; each component discharge,

m e a s u r e d i n t e n t h s of m i l l i s e c o n d s , a r e u s u a l l y three or four

is called a stroke.

strokes per flash,

s e p a r a t e d b y t e n s of m i l l i s e c o n d s .

is called a

whose luminous phase is There

the s t r o k e s b e i n g

Often lightning as

observed

0-8412-0481-0/79/47-096-079$12.25/0 ©

1979 A m e r i c a n C h e m i c a l Society

Scott; Toxic Chemical and Explosives Facilities ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

TOXIC CHEMICAL AND EXPLOSIVES FACILITIES

80 b y the e y e a p p e a r s

to f l i c k e r .

In t h e s e c a s e s t h e e y e

t i n g u i s h e s the i n d i v i d u a l s t r o k e s lightning stroke

dis-

which make up a flash.

Each

begins with a weakly luminous p r e d i s c h a r g e ,

the l e a d e r p r o c e s s ,

which propagates

f r o m c l o u d - t o - g r o u n d and

w h i c h is f o l l o w e d i m m e d i a t e l y by a v e r y l u m i n o u s r e t u r n which propagates

stroke

from ground-to-cloud.

It h a s b e e n f o u n d t h a t t h e e l e c t r o s t a t i c f i e l d t a k e s a b o u t 7 seconds

to r e c o v e r

to i t s p r e d i s c h a r g e v a l u e a f t e r t h e


of a l i g h t n i n g f l a s h at a d i s t a n c e

beyond 5 k m ,

occurrence

but w h e n the f l a s h

i s v e r y n e a r , t h e r e c o v e r y t i m e m a y b e d i f f e r e n t d u e t o the

pre-

s e n c e of s p a c e

field

takes

place

charge.

In b o t h c a s e s ,

regeneration

of the

exponentially.

Stepped L e a d e r .

The usual cloud-go-ground discharge

bably begins as a l o c a l d i s c h a r g e the c l o u d b a s e a n d the N - c h a r g e discharge frees

electrons

zed by attachment

pro-

region in

r e g i o n a b o v e it ( F i g u r e

1).

This

i n the N - r e g i o n p r e v i o u s l y i m m o b i l i -

to w a t e r

o v e r r u n the p - r e g i o n ,

b e t w e e n the p - c h a r g e

or ice p a r t i c l e s .

n e u t r a l i z i n g its

The free

electrons

s m a l l positive charge,

then continue their t r i p toward g r o u n d , w h i c h takes

about

msec.

to e a r t h

T h e v e h i c l e f o r m o v i n g the n e g a t i v e

charge

and

20 is

the s t e p p e d l e a d e r w h i c h m o v e s f r o m c l o u d - t o - g r o u n d i n r a p i d l u m i n o u s s t e p s a b o u t 50 m l o n g , leader

step o c c u r s

in less

as

shown in F i g u r e

than a m i c r o s e c o n d ,

1.

Each

a n d the t i m e

be-

t w e e n s t e p s i s a b o u t 50 Jj s e c . Return Stroke.

W h e n the s t e p p e d l e a d e r

relatively large negative tive c h a r g e

is near

surface

attract each other,

j o i n the l a r g e n e g a t i v e going d i s c h a r g e s .

(Figure

2).

Since

negative

charges

ground,

causing large propagates

100 M s e c .

W h e n the l e a d e r

to

determines

is attached

currents

to g r o u n d ,

to f l o w at g r o u n d a n d c a u s i n g

g r o u n d to b e c o m e v e r y l u m i n o u s .

The channel

c o n t i n u o u s l y u p the c h a n n e l a n d out

at a v e l o c i t y s o m e w h e r e b e t w e e n

the s p e e d of l i g h t . ctrons

attempts

and i n doing so initiates u p w a r d -

a t t h e b o t t o m o f t h e c h a n n e l m o v e v i o l e n t l y to

channel branches about

pro-

O n e of t h e s e u p w a r d - g o i n g d i s c h a r g e s

the l i g h t n i n g s t r i k e p o i n t .

the c h a n n e l n e a r

its

of p o s i -

opposite

the l a r g e p o s i t i v e c h a r g e

charge,

c o n t a c t s the d o w n w a r d - m o v i n g l e a d e r a n d t h e r e b y

luminosity

ground,

induces large amounts

o n the e a r t h b e n e a t h it a n d e s p e c i a l l y o n o b j e c t s

j e c t i n g a b o v e the e a r t h ' s charges

charge

1/2

T h e t r i p between ground and cloud

W h e n the l e a d e r

initially touches

f l o w to g r o u n d f r o m t h e c h a n n e l b a s e

and as

s t r o k e m o v e s u p w a r d , l a r g e n u m b e r s of e l e c t r o n s

the

and

1/10

takes

ground, the

f l o w at


ele-

return


4.

BENT

Lightning

and the Hazards

• • 7 / • • / • / 7 • • Figure 1.

Produces

81

/ s * s ss S > S ' * > y ^

V*">

Stepped leader initiation, (a) Cloud charge prior to p-N discharge, (b) stepped leader moving downward in 50-m steps.

*s s S ottom) normalized waveforms of the return stroke magnetic field for individual storms. ( ) Average of all storms, ( ) extreme average storm values.



4.

BENT

Lightning

and the Hazards

It

Produces

87

gap s p a c i n g i n m e t e r s

Figure 6.

Switching impulse breakdown voltage for H/D

i 0

I 20

1 40

I 60

: 80

i 100

i 120

= 1, which is average

I 140

— I 160

current, kA Figure 7.

Variation of striking distance with current amplitude



88 m e t e r s above ground.

These results,

therefore,

provide

q u a n t i t a t i v e e v i d e n c e a g a i n s t the b e l i e f i n l i g h t n i n g

concentration

areas. The

L i g h t n i n g R o d a n d Its The

Lightning Rod.

Conductors It i s a c o m m o n m i s c o n c e p t i o n t h a t

l i g h t n i n g r o d s d i s c h a r g e c l o u d s a n d thus p r e v e n t l i g h t n i n g .


rod only serves

as a m e a n s

g r o u n d b y d i v e r t i n g the l i g h t n i n g w h e n i t a p p r o a c h e s d i s t a n c e d i s c u s s e d i n the p r e v i o u s s e c t i o n . years

the

striking

In t h e t w o h u n d r e d

since B e n j a m i n F r a n k l i n investigated lightning,

manufacturers

many

h a v e t r i e d t o i n f l u e n c e t h e p u b l i c i n the

dissi-

p a t i o n p r i n c i p l e of l i g h t n i n g p r o t e c t i o n o r e l i m i n a t i o n . technique m o s t

The

to r o u t e t h e l i g h t n i n g h a r m l e s s l y to

c e r t a i n l y d o e s not w o r k a n d the

p h y s i c i s t s ' thoughts on this subject a r e f a s h i o n b y G o l d e i n the f o l l o w i n g

This

lightning

discussed in masterly

statement:

"It i s a m a n i f e s t a t i o n o f h u m a n w e a k n e s s

that a

pre-

j u d i c e o n c e a c q u i r e d t e n d s to b e r e t a i n e d e v e n i n t h e f a c e of o v e r w h e l m i n g f a c t u a l e v i d e n c e c o n t r a d i c t i n g the b a s i s o n w h i c h i t w a s f o u n d e d .

In t h e r e a l m o f

s c i e n c e a p r e j u d i c e m a y be t e r m e d a m i s c o n c e p t i o n . Such a m i s c o n c e p t i o n w h i c h has p e r s i s t e d for over hundred years

l i e f that a l i g h t n i n g c o n d u c t o r h a s the p u r p o s e ,

the a b i l i t y ,

There are

charge building

struck". several manufacturers

or lightning d i s s i p a t i o n s y s t e m s . belief,

be-

or indeed

of d i s s i p a t i n g s i l e n t l y the e l e c t r i c

i n a t h u n d e r c l o u d thus p r e v e n t i n g the " p r o t e c t e d " being

two

a n d w h i c h i s s t i l l w i d e s p r e a d i s the

however,

of e i t h e r r a d i o a c t i v e

lightning

The predominant scientific

i s that n e i t h e r of t h e s e s y s t e m s

are

any bene-

f i t o v e r the c o n v e n t i o n a l l i g h t n i n g p r o t e c t i o n s y s t e m .

Extensive

studies have been p e r f o r m e d r e c e n t l y on d i s s i p a t i o n a r r a y s which only serve

to e n h a n c e what s c i e n t i s t s

F r a n k l i n o v e r 200 y e a r s arrays

do no m o r e

p r o b a b l y do l e s s .

ago have

d a t i n g b a c k to

said; namely,

that t h e s e

than a c o n v e n t i o n a l lightning r o d and i n d e e d T h e s e s t u d i e s h a v e e x a m i n e d the h i s t o r i c a l ,

t h e o r e t i c a l a n d e x p e r i m e n t a l a s p e c t s of the a r r a y s i n v e s t i g a t e d the a r r a y s The arrays

are

and have

b e l i e v e d b y m a n y n o n - s c i e n t i s t s to g i v e o f f

significant corona discharge under thundercloud conditions that the e l e c t r i c a l c h a r a c t e r i s t i c s changed.

of the s t o r m a r e

because

this s o - c a l l e d e x c e s s i v e

so

drastically

T h e c l a i m s i n d i c a t e that a l a r g e a r e a a r o u n d the

is protected

also

o n s i t e at s e v e r a l i n s t a l l a t i o n s .

ionization


array

either


4.

BENT

Lightning

and

the

Hazards

It

Produces

89

r e a c h e s the c l o u d and l o w e r s i t p o t e n t i a l below the d i s c h a r g e l e v e l or p r o v i d e s a p r o t e c t i v e i o n c l o u d o v e r the p r o t e c t e d a r e a . T h e s e c l a i m s cannot be so as s u c h a p r o t e c t i v e c l o u d would have to c o n t a i n enough c h a r g e to m a k e i t m o r e d a n g e r o u s to the ground than the t h u n d e r c l o u d i t s e l f . Secondly, c o r o n a ions d i s s i pated f r o m an a r r a y would r e c o m b i n e i n n o r m a l a i r when o n l y a few h u n d r e d feet above the ground, and t h e r e would a l s o be l e s s ions f r o m an a r r a y than f r o m a single c o n v e n t i o n a l r o d or even a few t r e e s . A l l f a c t o r s r e l a t e d to these a r r a y s i n d i c a t e that they a r e not as good a l i g h t n i n g p r o t e c t o r as a c o n v e n t i o n a l s i n g l e c o n d u c t o r . A U.S. N a v y r e p o r t (4) and F A A r e p o r t (5) d i s c u s s both s i d e s of the topic. In o r d e r to e x a m i n e the c l a i m s r e l a t e d to r a d i o a c t i v e l i g h t n i n g r o d s , i t i s n e c e s s a r y to c o n s i d e r the p h y s i c a l p r o c e s s of a d i s c h a r g e to a c o n v e n t i o n a l r o d . When a l i g h t n i n g r o d i s i n the a r e a of a l i g h t n i n g l e a d e r the e l e c t r i c f i e l d a r o u n d i t s t i p would be e x t r e m e l y h i g h and the a i r i n this r e g i o n would be i n glow d i s c h a r g e w h i c h i n d i c a t e s m i l l i o n s of f r e e e l e c t r o n s m o v i n g at the point. A s the e l e c t r i c f i e l d i n c r e a s e s , this i o n i z a t i o n p r o c e s s or c o r o n a c u r r e n t a l s o i n c r e a s e s to a r c d i s c h a r g e and a s p a r k r e a c h e s out to m e e t the d o w n w a r d c o m i n g l e a d e r , f o r m i n g a path f o r e n o r m o u s c u r r e n t s to flow. Radioactive rods contain a c e r t a i n amount of radioactivity in the a r e a n e a r the t i p of the r o d , s u p p o s e d l y to enhance the i o n i z a t i o n and hence a t t r a c t the l i g h t n i n g l e a d e r o v e r l a r g e r d i s t a n c e s . T h e s e c l a i m s have been e x a m i n e d e x p e r i m e n t a l l y and t h e o r e t i c a l l y by m a n y s c i e n t i s t s with n e g a t i v e r e s u l t s . In effect, the a n a l y s i s shows the c o r o n a c u r r e n t f r o m the r a d i o a c t i v e r o d i s s l i g h t l y h i g h e r than that f r o m the c o n v e n t i o n a l r o d , as the m a n u f a c t u r e r c l a i m s , only when e l e c t r i c fields a r e low s u c h as u n d e r a f a i r sky. When a t h u n d e r h e a d a p p r o a c h e s and the e l e c t r i c f i e l d s b u i l d up, h o w e v e r , the r a d i o a c t i v e r o d gives off l e s s c o r o n a c u r r e n t than the c o n v e n t i o n a l r o d and i s , t h e r e f o r e , l e s s l i k e l y to be s t r u c k . T h i s c a n be e x p l a i n e d by the fact that the i o n i z a t i o n c l o u d p r o d u c e d a r o u n d the r o d by the r a d i o a c t i v e s o u r c e p r o v i d e s an i o n s h i e l d a r o u n d the t i p r e d u c i n g i t s e f f e c t i v e n e s s i n sending up the n e c e s s a r y u p w a r d l e a d e r s p a r k . T h e c o r o n a d i s c h a r g e f r o m a c o n v e n t i o n a l r o d was found to e x c e e d that f r o m a r a d i o a c t i v e r o d by an o r d e r of m a g n i t u d e u n d e r l i g h t n i n g - l i k e e l e c t r i c f i e l d s i n d i c a t i n g that r a d i o a c t i v e r o d s a r e m u c h l e s s c a p a b l e of i n f l u e n c i n g the path of a l i g h t n i n g d i s c h a r g e than a c o n v e n t i o n a l r o d . A n e x a m p l e of f a i l u r e of r a d i o a c t i v e l i g h t n i n g r o d s was i l l u s t r a t e d on the V a t i c a n ' s B e r n i


90


Colonade building in R o m e which is protected the 6th of M a r c h ,

1976,

the

Papal C r e s t was

by such r o d s .

a n d k n o c k e d o f f i n d i c a t i n g f a i l u r e of s u c h a p r o t e c t i v e T h e lightning r o d has lightning rods are common-sense

system.

the p u r p o s e of i n t e r c e p t i n g a

s t r i k e a n d d e f l e c t i n g i t f r o m the

structure.

to b e p u t o n a b u i l d i n g ,

On

struck by lightning

When

lightning

several

one s h o u l d d e v e l o p a

solution which will strike a reasonalble

balance

between protection and cost.


It i s p o s s i b l e , w i t h c a r e ,

to u s e

existing gutter and r a i n

p i p e s to o b t a i n p r o t e c t i o n at r e d u c e d c o s t s , taken when i n c o r p o r a t i n g m o d e r n m e t a l i c p a r t of the s y s t e m .

Lightning can penetrate

1 m m thickness or m o r e , m i g h t be a c c e p t a b l e . tain codes

but c a r e m u s t metal

sheets

a n d p e r h a p s the c o s t of s u c h

to b e 0. 3 m m f o r c o p p e r ,

repair

a n d 0. 5 m m f o r o t h e r thickness.

Down Conductors.

When lightning strikes an air

some

terminal

b y the s h o r t e s t

possible but

the i n d u c t a n c e of t h i s d o w n c o n d u c t o r i s a m a j o r of the d a n g e r o u s

i n t e r n a l grounded object,

pedence

cause

effects.

T h e down conductor has this function,

i n d e t e r m i n i n g the o c c u r r e n c e

strike

but c a n

l a r g e a r e a s of f o i l to be t o r n o f f d u e to the m e c h a n i c a l

the i n j e c t e d c u r r e n t m u s t be t r a n s f e r r e d

metals.

A lightning

to t h i s t y p e of r o o f w i l l n o t o n l y b u r n a l a r g e h o l e ,

because

to b e

of

The m i n i m u m thickness is defined in cer-

S o m e r o o f s u s e m e t a l f o i l s of l e s s

p a t h to g r o u n d .

be

roofing materials

factor

s i d e - f l a s h to

it m u s t a l s o h a v e the l o w e s t i m -

that c a n be a f f o r d e d .

T h e i n d u c t a n c e of a d o w n c o n d u c t o r i s d i r e c t l y p r o p o r t i o n a l to i t s h e i g h t .

B y p a r a l l e l i n g two d o w n c o n d u c t o r s

their combined

i n d u c t a n c e i s r e d u c e d to a p p r o x i m a t e l y o n e - h a l f that of a

single

conductor and so on.

spaced

too c l o s e

together

accurate.

The down conductors

however,

The importance

is therefore

otherwise

s h o u l d not be

the a b o v e r u l e i s not

of h a v i n g at l e a s t

a considerable advantage

two d o w n

i n r e d u c i n g the

s i d e - f l a s h , the a c t i o n of w h i c h i s d i s c u s s e d l a t e r . bends in a down conductor also i n c r e a s e s

conductors dangerous

Right angle

the i n d u c t a n c e a n d

such

a design needs careful consideration. Once a lightning strike has the s u r f a c e discharge are

of the e a r t h , the c u r r e n t

been intercepted and passed

i n t o the g r o u n d .

the g r o u n d r e s i s t a n c e ,

Two important

ground resistivity is high, ground.

to

factors

which plays a part in side-flashing,

a n d the p o t e n t i a l d i s t r i b u t i o n o v e r the g r o u n d s u r f a c e . the d o w n c o n d u c t o r s

to

i t i s the f u n c t i o n of e a r t h e l e c t r o d e s

advantages

to w a t e r

If t h e

c a n be a c h i e v e d b y b o n d i n g

p i p e s to l o w e r the r e s i s t a n c e

T h e r i s k i n s i d e - f l a s h i n g is thus d e t e r m i n e d

to

exclusively


4.

BENT

b y the

Lightning

and

the Hazards

It

91

Produces

inductance.

Side-flashing can also occur

b e l o w t h e g r o u n d to

buried

m e t a l p i p e s o r w i r e s a n d c a r e m u s t be t a k e n i n the d e s i g n a n d p o s i t i o n i n g of the g r o u n d i n g e l e c t r o d e s .

Typical values

pulse breakdown in soil are

2 to 5 k V / c m ,

flashes

In a i r

of s e v e r a l m e t e r s .

which leads

the v a l u e i s 9 k V / c m

and c o n c r e t e has a slightly lower b r e a k d o w n It i s i n t e r e s t i n g to n o t e


much more

of i m -

to

side-

and b r i c k

strength.

t h a t t h e l e n g t h of a g r o u n d r o d h a s

significant effect

o n the r e s i s t a n c e

C u r v e s d e m o n s t r a t i n g this effect are

than its

s h o w n i n F i g u r e 8,

which

a l s o i m p l i e s that l i t t l e b e n e f i t i s a c h i e v e d b y e x t e n d i n g the beyond 2 or

3 meters

Strip electrodes

are

exists below a layer Materials. ductors

or i n c r e a s i n g its d i a m e t e r

T h e t y p e of m a t e r i a l u s e d f o r r o o f a n d d o w n c o n -

to w h e t h e r

stranded f o r m .

the c o d e s code.

the m a t e r i a l

Stranded copper

C o p p e r or copper

A

strong corrosive off c o p p e r

be a v o i d e d a s

Copper,

but t h e r e a r e

aluminum conflicting

s h o u l d b e of r o d ,

tube,

strip

is not d e e m e d a c c e p t a b l e

although it is accepted

a l l o y s m u s t not be u s e d on a

in

i n the U S A building

fittings,

lead which are

effect

c a n be c a u s e d

conductors

onto s o m e

often u s e d on b u i l d i n g s . f a r as

stranded materials solid

a l l acceptable

of s e v e r a l c o u n t r i e s ,

with a l u m i n u m dripping

cm.

of l o w r e s i s t i v i t y .

and g a l v a n i z e d steel are or

rod

b e y o n d 1. 25

beneficial where high resistivity ground

s e e m s to be g o v e r n e d b y t r a d i t i o n .

opinions as

a

radius.

possible, are

more

by

rainwater

metals

such as

zinc or

Dissimilar metals

a n d one

s h o u l d be a w a r e

severely attacked

should

that

by c o r r o s i o n

than

conductors. C o r r o s i o n plays a high risk underground, in particular

a l u m i n u m which is totally unacceptable. perties

of s o m e

stray currents systems

where

The• e l e c t r o l y t i c

s o i l s c a u s e c o r r o s i o n to a l l t h e s e m e t a l s , p r o d u c e d by D C r a i l w a y l i n e s on D C h i g h the

earth is u s e d as

a return path.

to

proas

do

voltage

Cathodic pro-

t e c t i o n c a n h e l p e l i m i n a t e t h i s t y p e of p r o b l e m . The

Basic Requirements

of L i g h t n i n g

Protection

T h e r e l a t i v e n e e d f o r l i g h t n i n g p r o t e c t i o n at a f a c i l i t y i s pendent on m a n y f a c t o r s as or usage

indicated by Smith

(a)

Type

(b)

Personnel

safety;

(c)

Prevalence

of l i g h t n i n g ;

(d)

T y p e of

(6):

of f a c i l i t y ;

construction;


de-



92

0

I 1

| 2

I 3

i 4

L 5

length, m Figure 8.

Variation of ground resistance of rod electrodes of different diameter with length (British code)


4.

BENT

Lightning

and the

(e)

Contents;

(f)

Economic

(g)

Degree

Hazards

It

Produces

93

risks;

of i s o l a t i o n ( r e l a t i v e

h e i g h t of s u r r o u n d i n g

structures); (h)

T y p e of t e r r a i n ;

(i)

H e i g h t of

and

structure.

T h e d e c i s i o n to p r o v i d e p r o t e c t i o n m a y be b a s e d p r i m a r i l y on one f a c t o r


sonnel meet

alone.

S o m e p o s s i b l e s i n g l e f a c t o r s c a n be

safety r e q u i r e m e n t s ,

reduced insurance

an imposed lightning protection Three basic

requirements

tection against direct

rates,

per-

or

m u s t be f u l f i l l e d to p r o v i d e p r o -

lightning

strikes

to a

structure:

(a)

A c o n d u c t i v e o b j e c t m u s t be p r o v i d e d to i n t e n t i o n a l l y

(b)

A p a t h m u s t be e s t a b l i s h e d that j o i n s t h i s o b j e c t

a t t r a c t the l e a d e r

stroke;

e a r t h w i t h s u c h a l o w i m p e d e n c e that the f o l l o w s it i n p r e f e r e n c e (c)

A low resistance b o d y of the

epted

set

codes

protection The

s t r i k e to e a r t h .

United States has

and that is

T h e p r i m a r y d i f f e r e n c e i n the

requirements

two n a t i o n a l l y a c c e p t e d

Protection Association's

( A N S I C 5 . 1),

Lightning

a n d the U n d e r w r i t e r ' s Protection

effective

of t h e s e two c o d e s

are

codes;

the

Protection

Laboratories

S y s t e m (Standard

b a b l y e q u a l l y u t i l i z e d on s t r u c t u r e s

Master

U L 96A).

The

quite s i m i l a r and are

pro-

t h r o u g h o u t the n a t i o n .

The

d i f f e r e n c e b e t w e e n the t w o i s that the M a s t e r

Label can

c e r t i f i e d u p o n b o t h a f a c t o r y i n s p e c t i o n a n d l a b e l i n g of the ing protection m a t e r i a l s

and upon p e r f o r m a n c e

spection by an authorized Lightning

be

lightn-

of a f i e l d i n -

inspector.

Protection by O v e r h e a d W i r e

D a n g e r f a c i l i t i e s of l a r g e

d i m e n s i o n s r e q u i r i n g the

p o s s i b l e p r o t e c t i o n s h o u l d be p r o v i d e d w i t h a s y s t e m pass

acc-

es-

guidelines.

one t h i n g i n c o m m o n

i s the p h i l o s o p h y u s e d i n a c h i e v i n g a n

Labeled Lightning

major

have been

system.

National Fire Code

s h o u l d c o n f o r m to a n

to p r o v i d e t h e n e c e s s a r y

requirement has

diverting a direct various

system

C o d e s and standards

tablished in many countries or

and

earth.

of g u i d e l i n e s .

E v e r y code

to a n y o t h e r ;

to

discharge

c o n n e c t i o n m u s t be m a d e w i t h the

The lightning protection

aries

to

code.

suspended f r o m tall masts. to g r o u n d s o m e d i s t a n c e

These

catenary

best

of

caten-

wires

a w a y f r o m the p r o t e c t e d

must

structure

s o t h a t t h e l i g h t n i n g c u r r e n t m a y g o to g r o u n d a t a d i s t a n t p o i n t .



94 The grounding wires strip conductors

should radiate

a w a y f r o m the s t r u c t u r e ,

overhead wires

s h o u l d be f a r

e n o u g h f r o m the

nate s i d e - f l a s h i n g w h i c h was d e s c r i b e d e a r l i e r t e c t i v e a n g l e s f r o m the w i r e s m u s t c o v e r Theoretical investigations vated

grounded structures are

figures.


elevated

Figures

to

a n d the

pro-

the b u i l d i n g

of t h e e l e c t r i c

structure.

Hence,

such leader

a n d the

field

thunderstorm. leader

if two w i r e s w e r e

thus p r o t e c t i n g

the s t r u c t u r e

p r o t e c t i o n a r e a i s a f u n c t i o n of w i r e Figure

spaced

160

att-

feet a p a r t

a p p r o a c h i n g g r o u n d i n that r e g i o n w o u l d be

to t h e w i r e s ,

three

9 a n d 10 s h o w t h e e q u i p o t e n t i a l l i n e s a r o u n d a n

10 i t c a n b e s e e n t h a t i f a w e a k l i g h t n i n g

to i t .

ele-

s u m m a r i z e d i n the f o l l o w i n g

c a m e t o w a r d g r o u n d w i t h i n 8 0 f e e t o f t h e w i r e i t w o u l d be racted

elimi-

field around

grounded w i r e as d r a w n by a c o m p u t e r ,

Figure

The

structure

l i n e s a r o u n d a g r o u n d e d w i r e a t 2 50 f e e t d u r i n g a From

be

a n d s h o u l d n o t be b o n d e d to the s t r u c t u r e .

under them.

size and height,

any

attracted The

as

shown in

11.

Photographs

t a k e n of l i g h t n i n g s t r i k i n g a w i r e s t r e t c h e d

a canyon in N e w M e x i c o by D r .

over

M o o r e , v e r i f y that it d i d i n d e e d

s t r i k e the w i r e u n d e r n e a t h o n v a r i o u s

occasions.

a strong lightning leader

t h e f u n c t i o n of t h e w i r e ,

approaches,

in a high corona discharge thereby attracting

state,

At times

i s to p r o v i d e t h e u p w a r d

the d o w n w a r d s t r o k e

to the

when then

leader

wire.

If t h i s t y p e of l i g h t n i n g p r o t e c t i o n i s c a r e f u l l y p l a n n e d , c h a n c e s of f a i l u r e w i l l be e x t r e m e l y stallation and maintenance

small,

however,

the i n -

p r o b l e m s of s u c h a s y s t e m m a y

be

considerable. Electrical,

M e c h a n i c a l and T h e r m a l

Electrical Effects. attracted m o r e side-flash.

N o l i g h t n i n g s t r i k e to a s t r u c t u r e

a t t e n t i o n i n the l a s t d e c a d e s

It h a s

provided in order

t h a n the

has

so-called

been e x a m i n e d r e p e a t e d l y and its d a n g e r s

i l l u s t r a t e d i n the t e c h n i c a l l i t e r a t u r e . has

Effects

Its

prevention must

to s t o p i n c i d e n t s i n w h i c h a p r o t e c t e d

been struck and a p e r s o n in

are

be

building

such a building injured.

A n i l l u s t r a t i o n o f t h e p r i n c i p l e s o f t h e c o n d i t i o n s l e a d i n g to the r i s k of a s i d e - f l a s h a r e

shown in a simple example

in F i g u r e

12. T h e i l l u s t r a t i o n s h o w s t h e o u t l i n e s of a b u i l d i n g w i t h a lightning conductor highest point.

p r o t e c t i n g the c h i m n e y w h i c h c o n s t i t u t e s

In t h e a t t i c i s a n e l e c t r i c a l

point or a w a t e r

w h i c h i n t u r n i s c o n n e c t e d d i r e c t l y o r i n d i r e c t l y to g r o u n d . i n m i n d that a n e l e c t r i c a l

supply is

the pipe Bear

s t i l l a l m o s t at g r o u n d p o t e n -


BENT

Lightning and the Hazards It Produces


4.

Figure 9. Equipotential lines around an elevated grounded wire


95



96

Figure 10.

Electric field lines in the vicinity of an elevated grounded wire


4.

BENT

Lightning

and the Hazards

It

Produces

97


height of wire (ft)

0

20

40

60

collection region radius Figure 11.

80

100

120

(ft)

Field line collection area as a function of wire size and height



98 tial as

f a r as

the e x t r e m e l y

high lightning voltages

are

con-

cerned. Let us now assume

t h a t the l i g h t n i n g c o n d u c t o r

c h i m n e y is struck by a lightning c u r r e n t current

i s t h e n d i s c h a r g e d a l o n g the r o o f c o n d u c t o r ,

down conductor

a n d i n t o the e a r t h

stitutes an inductance ctrode m a y

L,

electrode.

be d e s c r i b e d b y its e f f e c t i v e

The

the

single

T h i s path

w h i l e the i m p e d a n c e

T h e t o p of the l i g h t n i n g - p r o t e c t i v e


on the

of a m p l i t u d e i .

con-

of t h e e a r t h

ele-

ground resistance

system

R.

i s t h u s r a i s e d to a

p o t e n t i a l w i t h r e s p e c t to t r u e e a r t h w h i c h i s g i v e n b y : u = iR + For

the p u r p o s e

an intense

Ldi/dt

of a n u m e r i c a l e s t i m a t e ,

lightning current

g r o u n d r e s i s t a n c e of R =

10Q.

tical conductor

160 (j H p e r

is about

The inductance

of t h e f r o n t o f t h e l i g h t n i n g , a s be t a k e n a s above

50k a m p / / ^

g r o u n d is

10 m ,

100

assume

k a m p and a

of a s i n g l e

ver-

m a n d t h e r a t e of

reported

\j s e c .

1

we m a y

o f c r e s t v a l u e i = 100

rise

b y L l e w e l l y n (_1),

If t h e h e i g h t o f t h e

may

chimney

t h e t o p o f the l i g h t n i n g c o n d u c t o r

is

r a i s e d t o a p o t e n t i a l w i t h r e s p e c t to t r u e e a r t h w h i c h a m o u n t s = 10

u

= 10

6

5

10 + 10 "

x

+ 3. 2

x 10

6

X

1.6

x

x

10"

4

10

x

6

2 x 10

x

V = 4. 2 M V n e g l e c t i n g

to:

V

S

phase

differences. In c o n t r a s t ,

the i n t e r n a l g r o u n d e d w i r e o r p i p e r e m a i n s

g r o u n d p o t e n t i a l e v e n w h e n the h o u s e i s s t r u c k tial difference

of 4 . 2 M V i s s u d d e n l y i m p r e s s e d

lightning-conductor potential difference, lightning conductor

an electric breakdown occurs f r o m

The breakdown strength

voltage

c a n be t a k e n a s

900

elethat the

to t h e w a t e r t a n k ; a n d t h i s i s t e r m e d

flash.

the

If t h e

of the c l e a r a n c e D i s l e s s t h a n

at

poten-

between

s y s t e m a n d the i n t e r n a l p o i n t .

ctric breakdown strength

materialize.

s o t h a t the

a

side-

of a i r f o r a c h o p p e d i m p u l s e

k V / m , hence

S i m i l a r situations m a y

a 5m flash could

occur

if people a r e

standing

between a grounded a i r - t e r m i n a l lead and a grounded i n s t r u m e n t i n the b u i l d i n g . surface

great distances. ductor

Side-flashes may also occur

a n d i n the p r o c e s s c a n t h r o w u p r o c k s

Thermal Considerations. return

many

stroke 1-2

current

problems.

The lightning leader

cone w h i c h is s u r r o u n d e d by m u c h l a r g e r

is about

the

over

T h e s i m p l e s o l u t i o n of b o n d i n g the d o w n c o n -

to the g r o u n d e d o b j e c t w i l l a l l e v i a t e

narrow

under

and soil

is concentrated

centimeters diameter

stroke

corona.

in this c e n t r a l

and a m a x i m u m

has

a

The

cone which

temperature



4.

BENT

Lightning

and the Hazards It

Produces

99

Lightning Conductor

E l e c t r i c a l Supply

Water

Pipe

Figure 12.

Lightning strike to house and conductor showing distance to interior grounded unit

fi 6t, 2

Figure 13.

2

A s

Temperature rise in copper conductors (after Golde (3))


100


of a b o u t 60, 000

°F is reached after

T h e effects serious,

although there

w i l l be p e n e t r a t e d . i t, 2

on m e t a l s

is u s u a l l y not

is a p o s s i b i l i t y that a thin m e t a l

The temperature

rise

sheet

i s p r o p o r t i o n a l to

w h e r e a m a x i m u m v a l u e o f ^ i t d t i s a b o u t 10 ^ a m p 2

2

sec.

N e g l e c t i n g d i s s i p a t i o n a n d r e f e r r i n g to F i g u r e

13,

the t e m p e r a t u r e

specified in most

lightning codes

rise as

the t e m p e r a t u r e


a few m i c r o s e c o n d s .

of t h i s t e m p e r a t u r e

of c o p p e r c o n d u c t o r s ,

30 t o 50 m m

2

,

as

is m o d e r a t e .

v a l u e s c a n be t a k e n as

1.5

one s e e s

For

that

aluminum

times those

for

copper. T h e r e is one a s p e c t sideration.

of h e a t d i s s i p a t i o n that n e e d s

through a high resistance lapping metal sheets, sparking.

con-

W h e r e the l i g h t n i n g c u r r e n t is b e i n g d i s c h a r g e d joint,

the h e a t

such as a poor contact

or

over-

g e n e r a t e d m a y g i v e r i s e to h e a v y

P e n e t r a t i o n m a y o c c u r i n t h e c a s e of t h i n m e t a l

such as u s e d in roofing m a t e r i a l or a i r c r a f t the h o l e i s a f u n c t i o n o f l i g h t n i n g c h a r g e , thickness.

For

20 m i l c o p p e r ,

skin.

sheets

The size

of

the m a t e r i a l a n d i t s

the h o l e c o u l d be u p to 300 m m

2

.

W h e n l i g h t n i n g s t r i k e s a n i n s u l a t i n g m a t e r i a l the p o i n t of contact

c o u l d be r a i s e d to a h i g h t e m p e r a t u r e

could result.

B y these

means

and penetration

c l e a n h o l e s of 2 c m d i a m e t e r

been punched i n glass by lightning d i s c h a r g e s .

have

If t h i s i n s u l a n t

c o n t a i n s m o i s t u r e the c u r r e n t w i l l f l o w p r e f e r e n t i a l l y a l o n g the p a t h of best c o n d u c t i v i t y . and explosions o c c u r .

M o i s t u r e c a n be c o n v e r t e d into

E n o r m o u s b l o c k s of c o n c r e t e

have

steam been

d e m o l i s h e d this w a y and on one o c c a s i o n r o c k y g r o u n d w a s r o w e d f o r 800

f e e t a n d 75 t o n s

explosive effect was

e q u i v a l e n t to 6 0 0

Mechanical Considerations. shock wave and bending f o r c e s . creases discussed earlier, pands extremely wave.

Figure

central cone, for thunder,

14 as

fur-

of r o c k a n d s o i l d i s l o d g e d .

The

l b s of T N T .

M e c h a n i c a l effects

concern

W i t h the r a p i d t e m p e r a t u r e

the a i r

s u r r o u n d i n g the c h a n n e l

rapidly and produces a supersonic

ex-

pressure

shows how this wave is propagated f r o m c a l c u l a t e d b y H i l l (7_).

the in-

the

It i s r e s p o n s i b l e n o t

b u t a l s o f o r w i d e s p r e a d l i f t i n g of t i l e s o n the

only

roofs

of b u i l d i n g s . Two parallel conductors s u b j e c t to a t t r a c t i v e

caught in a lightning d i s c h a r g e

f o r c e s and these f o r c e s are

are

responsible for

t h e f u s i n g of s t r a n d e d c o n d u c t o r s a n d f o r s q u a s h i n g h o l l o w c o n ductors. T h e r e i s one m o r e m e c h a n i c a l f o r c e w o r t h c o n s i d e r i n g . a lightning conductor follows a right-angle

If

bend on a b u i l d i n g and

t h i s c o n d u c t o r h a s to d i s c h a r g e a l i g h t n i n g c u r r e n t ,

it w i l l be


BENT

Lightning

and the Hazards

1

0


4.

It

Produces

2

3

101

5

4

radius, cm Figure 14.

D

J

Development of pressure from lightning channel

F

M

A

M

J

J

A

S

O

N

D

MONTHS

Figure 15.

Annual variation of thunderstorm activity in terms of flashes to ground in Orlando, Florida



102 subject

to a m e c h a n i c a l f o r c e

attempting

p r o p o r t i o n a l to t h e strokes

t r y i n g to s t r a i g h t e n i t a n d

to b e n d i t o u t w a r d . square

of t h e c u r r e n t ,

i t c a n o n l y r e a c h a b o u t 5, 000

bends i n conductors


3 per minute.

Sharp

rectangular

T h e d u r a t i o n of a

of 30 m i n u t e s ,

thunder-

with a flashing rate

Since a s t o r m contains

c e l l s at a n y i n s t a n t ,

is

large

be a v o i d e d .

F r e q u e n c y of S t r i k e s to G r o u n d . 2 or

but e v e n f o r

lbs.

should, therefore,

s t o r m c e l l i s of the o r d e r

thus,

T h e m a g n i t u d e o f the f o r c e

one o r two

3 to 4 f l a s h e s p e r m i n u t e i s n o t

unreason-

a b l e a s a n a v e r a g e w i t h a s t o r m d u r a t i o n of a p p r o x i m a t e l y hour.

T h e p r o p o r t i o n of d i s c h a r g e s storm.

follows,

over

therefore,

intracloud discharges

a possible occasional increase in a s e v e r e c a s e . Pierce

1 to 4 .

of

It

Figure

rate for flashes

round

to g r o u n d w i t h

to one e v e r y t h r e e o r f o u r

15 i l l u s t r a t e s

to g r o u n d f o r O r l a n d o ,

as

seconds

the a n n u a l v a r i a t i o n

calculated

of

by Cianos and

(8).

The

s p a t i a l d i s t r i b u t i o n of f l a s h e s

in lightning p r o b l e m s . well separated, observer

c a n be a s i g n i f i c a n t f a c t o r

T h e f a c t that c o n s e c u t i v e

often q u a s i - r a n d o m l y ,

of t h u n d e r s t o r m s .

c e p t that the d i s c h a r g e s still

is

that one p e r m i n u t e i s a n a p p r o p r i a t e

f i g u r e e s t i m a t e of t h e o c c u r r e n c e

discharges

o f the

T y p i c a l l y w i t h i n the U n i t e d S t a t e s the r a t i o

cloud-to-ground

one

that go to g r o u n d i s q u i t e

v a r i a b l e f r o m s t o r m to s t o r m a n d a l s o d u r i n g p h a s e s same

of

active

Nevertheless,

progress

flashes

are

i s f a m i l i a r to a n y

careful

the e r r o n e o u s

in a steady

con-

orderly pattern

is

prevalent. The only thunderstorm

statistics which are

a b l e i s the t h u n d e r s t o r m d a y .

readily avail-

A day is defined m e t e o r o l o g i c a l l y

a s a t h u n d e r s t o r m d a y i f t h u n d e r is h e a r d ; t h i s i m p l i e s the currence

of l i g h t n i n g w i t h i n a b o u t

No account

15 k m o f t h e o b s e r v i n g

oc

site.

i s t a k e n i n the t h u n d e r s t o r m d a y s t a t i s t i c of the

n u m b e r of t i m e s

thunder is h e a r d ,

t h u n d e r s t o r m events

per

n o r the n u m b e r of

thunderstorm day.

Figure

discrete 16

illus-

t r a t e s t h e n u m b e r of t h u n d e r s t o r m d a y s r e c o r d e d i n t h e U S A , a n d s h o w i n g a n e x c e s s of 100 s u c h d a y s i n t h e S t .

Petersburg

O r l a n d o r e g i o n o f F l o r i d a w i t h a v a l u e o f a b o u t 80 a t

to

Cape

Canaveral. C i a n o s and P i e r c e

give a useful relationship for d e t e r m i n i n g

the f r e q u e n c y o f s t r i k e s u n d e r a t h u n d e r s t o r m . a v a i l a b l e by w h i c h data for T of Q , n

n

the f l a s h i n c i d e n c e p e r k m

2

per month,

g r o u n d f l a s h i n c i d e n c e (pO ) p e r k m m

b y the p r o p o r t i o n p of f l a s h e s

Two methods

c a n be c o n v e r t e d 2

are

estimates

and hence,

per month,

that go to e a r t h .

into

the

by multiplying T h e two


methods


4.

BENT

Lightning

Figure 16.

and the Hazards

It

Produces

103

The average number of days per year on which thunder is heard in various parts of the USA

M o n t h l y Dependence

of T h u n d e r s t o r m

Activity

C a p e K e n n e d y 195 7- 1962 (P = 0.

Month T January

P CT

m

. 5

li n

. 12

. 02

February

1.8

.25

. 05

March

3, 7

. 53

. 10

April

3. 3

.45

. 08

May

6.7

1.42

. 26 1. 06

June

14. 0

5.91

July

13.8

5 . 75

1. 04

15. 8

7 . 52

1. 35

10. 5

3 . 35

.60

August September October

3.8

. 55

. 10

November

0. 7

. 15

. 03

December

0. 7

. 15

. 03



104

d o not d i f f e r g r e a t l y i n the r e s u l t s t h e y y i e l d n e a r (2 t o 10)

of T

m

of m o s t p r a c t i c a l i m p o r t a n c e .

the

Pierce

range concludes

that:

where a equals 3 x

10

2

= a T

2

a

.

B

+ a

T*

2

T a b l e 1 i l l u s t r a t e s these

C a p e K e n n e d y w h e r e the v a l u e s of p a r e

taken as

results

0.

at

18.

U s i n g t h e s e f o r m u l a s we c o n c l u d e that a p p r o x i m a t e l y f i v e


discharges

come

to g r o u n d e a c h y e a r p e r

square k m .

F r e q u e n c y of S t r i k e s to T a l l S t r u c t u r e s * s t r i k e s to t a l l s t r u c t u r e s t r o l l e d by two f a c t o r s .

O n e i s the a t t r a c t i v e

i s the t r i g g e r i n g f a c t o r . these

factors

cussed here.

T h e i n c i d e n c e of

e l e c t r i c a l l y c o n n e c t e d to g r o u n d i s P i e r c e and P r i c e

(9) h a v e

attractive

area A

height h.

The attractive

(= TTr

radius,

) are

r„ ,

and its

dis-

associated

p r i m a r i l y f u n c t i o n s of

r a d i u s is defined as

other

investigated

i n d e t a i l a n d a f e w of t h e i r f i n d i n g s w i l l b e The attractive

con-

r a d i u s a n d the

structure

the a v e r a g e

radius

at w h i c h a d o w n w a r d l e a d e r f r o m the c l o u d i s j u s t a b l e to i n d u c e an upward streamer

f r o m the s t r u c t u r e

that w i l l u n i t e w i t h

the

d o w n w a r d l e a d e r a n d t h u s d i v e r t t h e f l a s h t o the s t r u c t u r e . triggering factor

represents

t i a t e d at the t i p of the s t r u c t u r e ; but a s h i n c r e a s e s , c o m m o n and for

m

the t r i g g e r e d v a r i e t y of d i s c h a r g e

It i s p o s s i b l e to c a l c u l a t e

sentations

r

.

However,

the

calculations

c a n be c r i t i c i z e d i n m a n y r e s p e c t s .

Cianos

(8) h a v e g i v e n a c o m p l i c a t e d e x p r e s s i o n f o r r

f u n c t i o n of h .

m,

increasingly

important.

that h a v e b e e n m a d e and P i e r c e

ini-

i t i s n e g l i g i b l e f o r h ^ 100

triggered flashes become

h ^250

i s b y f a r the m o r e

The

the i n c l i n a t i o n of f l a s h e s to be

T h i s i s b a s e d b o t h o n the m a t h e m a t i c a l

emerging f r o m theoretical analysis,

p i r i c a l f i t w e i g h t e d a c c o r d i n g to t h e d e g r e e v a r i o u s data s o u r c e s . N o t e that a b o v e about

Table 2 shows r 150 m ,

l a t i o n s i n d i c a t e t h a t f o r h ^ 150 m ,

a

and on an

em-

of r e l i a b i l i t y of the

a s a f u n c t i o n of h .

the a t t r a c t i v e

change with a further height i n c r e a s e . t w e e n the t i p of the s t r u c t u r e

a

as

repre-

radius does

not

T h i s is because

calcu-

the f i e l d d i s t r i b u t i o n b e -

a n d the d o w n c o m i n g l e a d e r i s n o t

m u c h i n f l u e n c e d b y the p r e s e n c e

of the g r o u n d .

P i e r c e h a s p o i n t e d out that r e p o r t e d i n s t a n c e s

of t r i g g e r e d

l i g h t n i n g o c c u r w h e n the a m b i e n t g e n e r a l e l e c t r i c f i e l d E b e t w e e n 3 a n d 30 k V / m a n d t h e v o l t a g e d i s c o n t i n u i t y V

lies

,

b e t w e e n the t i p of the c o n d u c t o r c a u s i n g the t r i g g e r i n g a n d the unperturbed atmosphere,

i s 0. 3 t o 6 M V .

t h a t f o r t h e l o w e r v a l u e s of E

or V ^ there a D

It s e e m s p l a u s i b l e i s a s m a l l but f i n i t e


4.

BENT

chance

Lightning

and the Hazards

It

of l i g h t n i n g b e i n g t r i g g e r e d ;

be g r e a t e r t h e l o n g e r t h e v a l u e s As

E

a

Produces

and Vj^ increase

lightning,

this chance

of E

so w i l l the p r o b a b i l i t y of

but the c h a n c e

Table height.


cations

3 summarizes

T h e data

base

in C o l u m n

is

so scanty

Also

of E

d u e to H o r v a t h (10).

overestimates

lightning as

h ^ 150

m,

that s u b s t a n t i a l future

by P i e r c e

b e c o m i n g b e t t e r f o r h*%/400 As

an example,

Cape Canaveral.

theoretical expressions

Horvath's work much of h ,

and

E x p r e s s i o n (1) f i t s w e l l for large

(2) u n d e r e s t i m a t e s

h.

throughout,

but the a g r e e m e n t

is

m.

let u s c o n s i d e r a 260

foot o r 80 m

a n d T a b l e 3 i n d i c a t e s a n a v e r a g e v a l u e o f . 16 f o r t h e t r i g g e r e d to n a t u r a l l i g h t n i n g . Thus,

gives

for

tower

T a b l e 2 g i v e s the a t t r a c t i v e r a d i u s as

g r o u n d at the C a p e h a s

modifi-

the i n f o r m a t i o n

and some

the i n c i d e n c e at l o w e r v a l u e s

but o v e r e s t i m a t e s

Expression

avail-

a f u n c t i o n of

N o n e of the t h e o r e t i c a l

for high h.

length

exist.

1 thelbest presently

a g r e e w e l l w i t h the e x p e r i m e n t a l d a t a . underestimates

and

shown in Table 3 are

d e r i v e d f r o m two e x p r e s s i o n s results

maintained. triggered

w i l l a g a i n be d e p e n d e n t on the

on the i n c i d e n c e of t r i g g e r e d

could occur.

will obviously

a n d V-p a r e

a

of t i m e f o r w h i c h a n y s p e c i f i c v a l u e s able data

105

310

ratio

T h e i n c i d e n c e of f l a s h e s

b e e n s h o w n i n T a b l e 1 to be 4 .

the a n n u a l i n c i d e n c e of n a t u r a l l i g h t n i n g to the

should be,

at m

of

to

7/km . 2

tower

, 4. 7

TT

x

x

(310)

2

10'

x

=1.42

T r i g g e r e d lightning should contribute a further

incidence

of

some, 0. 16 x 1 . 4 2 T h e t o t a l n u m b e r of s t r i k e s the o r d e r

o f 1. 65 p e r

= . 23 to the t o w e r

Lightning and Switching Surges and Surge elements

will,

or transient voltages of e l e c t r i c a l

are

o n e of the l e a s t

transients

The random characteristics

appear

that

of p o s s i b l y

can exist

even after component damage

or

of t h e i r m a g n i t u d e a n d is still

software

They

unsuspected,

default.

c a n be a r a p i d c o m p o n e n t f a i l u r e o r t r a n s i e n t s the c o m p o n e n t c a u s i n g d e g r a d a t i o n ,

several

in all

d i f f i c u l t to i d e n t i f y a n d a n a l y z e .

unexpedtedly and their p r e s e n c e

very

These unwanted sub-

thousand volts and s e v e r a l hundred amps them

understood

simple reason

subject.

m i c r o s e c o n d t i m e - t o - p e a k voltage systems.

be o n

Transients

e n e r g y f o r the

l i t t l e d a t a i s a v a i l a b l e on the

duration make

therefore,

year.

can

The

damage

slowly attack

which can produce


random



106

T a b l e II.

Relation Between H e i g h t (h) a n d

h

r

(m)

III.

)

~

150

50

~

250

100

~350

150

~

400

~

400

P r o p o r t i o n of T r i g g e r e d to N a t u r a l L i g h t n i n g

Structure

Actual

Height

Data

(m)

Expression (1)

50

~

0

~

0

100

~

0

~

0

0. 3

~ 0

150

a

(m)

a

25

>150

ble

Structure

A t t r a c t i v e R a d i u s (r

Expression (2)

Horvath Theory 0. 1

~0

0. 2 0. 5

0.4 0. 7

200

1

0. 1

2. 8

300

4

1. 3

16

1.4

400

10

6

38

3. 0


4.

BENT

Lightning

erroneous failure

and

the Hazards

signals for

It

Produces

107

s e v e r a l weeks until total

T h e e l i m i n a t i o n of t h i s r a p i d e n e r g e t i c special devices or

overvoltage

power

surge

protectors

and

C o n s i d e r a b l e problems exist with filtering

unless they are

expected

insulation requirements

especially made and are

with m u c h higher

e x i s t s that t r a n s i e n t s

software

than

used in conjunction

specifically designed overvoltage

often c a u s e

gas d i o d e s ,

c o m m o n l y found in c o m p u t e r s

devices,

Evidence

requires

a n d c a n n o t be p r e v e n t e d b y c r o w b a r s ,

supplies.

with fast,


component

occurs.

protectors.

entering computer

systems

p r o b l e m s b y c a u s i n g b i t s to be c h a n g e d

b i t s to b e a d d e d t o m e m o r y l o c a t i o n s . a l s o b e e n s h o w n to b e c a u s e d

These transients

by c o m p u t e r

and

have

tape r e c o r d e r s

pro-

v i d i n g the i n d u c t i v e i m p u l s e . There are

three specific reasons

suppression requirements: lightning on s o l i d - s t a t e world weather semiconductor

(1)

components

pattern).

(2)

technology,

by a d e g e n e r a t i n g

in voltage

effect

of i n d u c e d

(also enhanced by a changing

M o r e and m o r e

sophistication in

s i m p l y s m a l l e r and s m a l l e r

(3) T h e v e r y m o n u m e n t a l e f f e c t caused

f o r the c h a n g e

The increased

of s w i t c h i n g

s u p p l y of c o m m e r c i a l p o w e r .

demand upon power companies

increases,

another,

loads are

constantly being

causing "surges"

short duration transients little as

one

same

cause high

speed,

o f e n e r g y a p p l i e d to

c a n c a u s e a s h u t d o w n of o p e r a t i o n s .

can either

rate.

s w i t c h e d f r o m one l i n e to

(which in turn,

i s a c o m p u t a t i o n b y O d e n b e r g (1JJ of e n e r g y

the

rate,

to p r o c e e d d o w n the p o w e r l i n e ) .

(1) n a n o j o u l e (1 x 10"^)

semiconductor

As

at a g e o m e t r i c

the a b i l i t y to p r o d u c e p o w e r d o e s n o t i n c r e a s e at the Therefore,

devices.

transients/surges

upset

As

the

Figure

17

showing how a s m a l l amount

or d e s t r o y a t r a n s i s t o r ,

I. C .

or

semiconductor. The major due

sources

of l i g h t n i n g

surges

in conductors

are

to a)

Ground potentials

b)

Induced effects a

caused

caused

by n e a r b y lightning

by lightning c u r r e n t

on

shield;

c)

Direct strikes

d)

S i d e - f l a s h e s to the c o n d u c t o r f r o m a n e a r b y

e)

A straight

to a w i r e ;

conductor

antenna for lightning f)

strokes;-

flowing

A looped conductor for lightning

acting as

strike;

an electrical field

change

effects;

a c t i n g as

a magnetic

field

antenna

effects.

B u r y i n g the c a b l e d o e s not r e m o v e

lightning effects,

c a b l e i s t h e n a n i d e a l g r o u n d p a t h f o r the c u r r e n t .

The


as

the

lightning

108


current may

s i d e - f l a s h s e v e r a l m e t e r s to the c o n d u c t o r

the g r o u n d ,

where

the d i s t a n c e

r e s i s t i v i t y a n d the r e s i s t a n c e of the c o n d u c t o r The largest line r e a c h e d

lightning voltage

recorded

i n F i g u r e 18, a n d t h e s t r i k e o c c u r r e d It i s

suggested

that c l o s e r

to the

transmission than

two

r e c o r d i n g is

shown

s o m e 4 m i l e s u p the

s t r i k e p o i n t the c u r r e n t

of r i s e w a s p r o b a b l y of the o r d e r


on a

The resulting oscilloscope

solid

to g r o u n d .

a p e a k v a l u e of 5 m i l l i o n v o l t s i n l e s s

microseconds.

under

is p r i m a r i l y a f u n c t i o n of

line. rate

o f 10 m i l l i o n v o l t s p e r

micro-

second. R e s i d e n t i a l 120V lightning associated transients nature, hertz

A C lines are

voltages

u p to 3 k V .

f o u n d to e x p e r i e n c e

peak

of u p to 6 k V a n d i n t e r n a l s w i t c h i n g

The transients

w i l l be o s c i l l a t o r y i n

w i t h a f u n d a m e n t a l f r e q u e n c y f r o m a f e w t e n s of k i l o -

to s e v e r a l m e g a h e r t z

h u n d r e d s of m e g a h e r t z . 100 m i c r o s e c o n d s

with components

ranging into

T h e y w i l l l a s t f r o m 100 n a n o s e c o n d s

a n d c a n be c l a m p e d w i t h i n a f e w c y c l e s .

grounding and bonding m a y reduce

the t r a n s i e n t s

to

Good

significantly.

I n t r a c l o u d l i g h t n i n g c a u s e s a c o n s i d e r a b l e n u m b e r of i n duced effects in cables hundred amps, discharge

of s e v e r a l t h o u s a n d v o l t s a n d

e v e n t h o u g h the

separation distance

m a y be s e v e r a l m i l e s .

e f f e c t i s that the p o w e r , antenna.

Shorter

f l e c t i o n s at the c a b l e

The main reason

telephone or data

cables

give rise

surges

heaters, Force

can cause several

such as

53,000 damaging surges

A n example nautical Radio,

i.e.,

solenoids

conditioners per day.

over

and At an A i r

1 joule were

of w h i c h 10, 000

re-

occurred

of i n d u c e d v o l t a g e s 1967).

w o u l d be a p e a k of

30 v o l t i n d u c t i v e l o a d

in

systems.

T h i s w o u l d be g e n e r a t e d

like elevators

in hospitals.

s a y s t h a t 110

transients.

"3900

(Aero-

T h e N a v y r e g a r d a 2. 5 t h o u s a n d v o l t p e a k

as a m a x i m u m i n d u c t i v e s w i t c h i n g t r a n s i e n t

Institute

re-

(Odenburg).

v o l t s " p r o d u c e d by a 4. 0 a m p , voltage

air

thousand surges

c o r d e d d u r i n g a one m o n t h p e r i o d , one d a y

an

due to

ends.

and local m o t o r s ,

site over

to

for such an

cable acts as

to l a r g e r

Transients f r o m switching inductive leads, and r e l a y s ,

several

of c a b l e

by large

o n 110 V A C

inductive switching,

The A m e r i c a n National Standards

volt line faults can cause six thousand volt

Sparks f r o m a charged human can also reach

two

t h o u s a n d v o l t s i n one n a n o s e c o n d a n d s e v e r a l t e n s of t h o u s a n d s volts

shortly

of

after.

T h e utilitie's induce. surges gizing transformers

while e n e r g i z i n g and

i n a n e f f o r t to m a n a g e

utilitie's p r o v i d e p r i m a r y p r o t e c t i o n ,

loads.

de-ener-

Although

the f r o n t e n d of s u r g e s


may

BENT

Lightning

and the Hazards

MOST SUSCFPTIBLE-JO 9 10

8

10

7


IJKJ

1 0

6

—

10

4

10

3

10'

D I G I T A L IC's

2

10

HIGH SPEED T R A N S I S T O R S . IC's

-B •

10

4

10

3

10

2

10

1

-

+

UPSET

LOW-POWER TRANSISTORS SIGNAL DIODES

_ M E D I U M POWER TRANSISTORS ZENERS AND RECTIFIERS _ HIGH-POWER TRANSISTORS _ POWER SCR's, POWER D I O D E S

1

1

1

LEAST SUSCEPTIBLE

LOW-NOISfc — TRANSISTORS AND DIODES

u

5

10-

Produces

10 '

10" 10-

1 —

It

10

+ 1

10

+ 2

-

BURNOUT

N o t e : f o r t r a n s i e n t s i n the m i c r o s e c o n d r e g i o n

Figure 18.

Oscillogram of voltage surge on a HOW transmission line

50

\00

300

TIME (^sec)

Figure 17.

Upset and burnout energies for various semiconductors



110 not be e l i m i n a t e d .

Also,

the e n e r g y

stored in transformers

e n e r g i z e d a f t e r the p o w e r c o m p a n y ' s a r r e s t o r s f i r e a n d power for

1/2

also appears

cycle can also cause damaging surges.

A

surge

at p o w e r r e - i n i t i a l i z a t i o n .

Another important source a t t e m p t i n g to w i r e , ients are

de-

remove

adjust or

of s u r g e s service

is human error.

systems,

While

unwanted

trans-

p r o d u c e d w h i c h m a y p r o v e to be d e t r i m e n t a l to

semi-


conductors. Insulation Effects.

T h e e f f e c t of h i g h v o l t a g e s

c a n be q u i t e l a r g e a n d w h e r e a s s u l a t o r m a y n o t be c a t a s t r o p h i c , voltage transients

the r e p e t i t i v e

effects

w i l l p r o d u c e b r e a k d o w n at the

u n t i l the i n s u l a t o r c a n n o t e v e n s t a n d the s t e a d y electric

clock manufacturer

on i n s u l a t i o n

the i n i t i a l b r e a k d o w n of a n i n of h i g h

same

place

state voltage.

r e d u c e d h i s f a i l u r e r a t e to

An

one

h u n d r e d t h of h i s e a r l i e r f a i l u r e r a t e b y i n c r e a s i n g the i n s u l a t i o n l e v e l f r o m 2 to 6 k V .

S u r g e p r o t e c t i o n w o u l d h a v e the s a m e

Breakdown will also occur along a surface circuit board.

In t h i s c a s e ,

a p a t h of s l i g h t l y c o n d u c t i v e

bonized insulation will occur vaporized metal.

Steep wavefront voltages

m a y l e a d to

grounded object.

or it m a y

enclosure are

where a cable

(12).

s h o u l d be kept as

w h i c h a l l o w s the c u r r e n t enclosure.

This example

possible.

enclosure

to f l o w r a d i a l l y f r o m t h e (Figure

This indicates

s h i e l d to

If a s e p a r a t e g r o u n d l e a d i s u s e d t o c a r r y

it w i l l have an inductance o f o n e i n c h o f 0. 0 3 4 "

diameter

a n d w i t h a r a p i d c u r r e n t p u l s e of 100 d e v e l o p e d w i l l be

1340

volts.

v o l t s to d e v e l o p b e t w e e n over

1 c m long.

19b) w h i c h w i l l l e a d

amp/nsec,

T e n inches

should,

The in-

w i r e i s a b o u t 0. 0 1 3 4 JJh the

voltage

of w i r e w i l l e n a b l e

shield and e n c l o s u r e ,

The wire

the

surge

to a p o t e n t i a l d i f f e r e n c e b e t w e e n s h i e l d a n d e n c l o s u r e . ductance

effects

shield and an

s h o r t and d i r e c t as

19a s h o w s a s h i e l d b e i n g t e r m i n a t e d o n a n

current,

grounds

m u s t be c a r r i e d out w i t h c a r e

19,

connected together

that g r o u n d l e a d s

deve-

of t h i s

The inductive

s t i l l l e a d to h a z a r d o u s p o t e n t i a l d i f f e r e n c e s .

p r o b l e m is i l l u s t r a t e d in F i g u r e

Figure

T h e effects

B o n d i n g the

w o u l d h a v e r e m o v e d the p r o b l e m .

of s u c h b o n d i n g b e t w e e n t w o o b j e c t s

is

large potentials m a y

p r o b l e m have been illustrated e a r l i e r . together

break-

coil.

W h e n one g r o u n d e d c o n d u c t o r

conducting a steep wavefront current, l o p b e t w e e n it a n d a n o t h e r

car-

w h i c h m a y a l s o be i n f l u e n c e d b y

d o w n o f i n s u l a t i o n b e t w e e n the w i n d i n g s of a Grounding and Bonding.

effect.

s u c h as a p r i n t e d

therefore,

implying a be k e p t as

possible.


13k

spark short

as


(a)

Figure 19.

DAOIAI FLOW OF CURMMT

1

(b)

coNCiMTAATto FLOW or CUMCNI

Connections between shield and enclosure; (a) good, (b) bad

CNCLOSUiC


I

vvC

crc> 3-

H

w w


112

It i s a l s o a b s o l u t e l y e s s e n t i a l t h a t a s h i e l d b e g r o u n d e d a t both ends in short l i n e s .

The magnetic fields caused

by

lightning can induce voltages around open c i r c u i t loops and currents

around short circuit loops.

hardly ever

cause damage,

c e s s i v e l y high and will cause Ideally,

one m u s t

The induced


crowbar.

devices are

for

the

separate

point.

of two b a s i c t y p e s ,

state v o l t a g e ,

will conduct v e r y heavily.

voltage

This

i s b r o u g h t to a l o w l e v e l .

will never reduce

very

but a b o v e a c e r t a i n v o l t a g e

A c r o w b a r device in effect,

c i r c u i t s a h i g h v o l t a g e to g r o u n d . the c u r r e n t

constant

T h e constant voltage devices will conduct

l i t t l e at the s t e a d y

but

ex-

Devices

Protector and

systems

of a s y s t e m a n d c o m b i n e these

g r o u n d s at o n l y one c o m m o n r e f e r e n c e Protector

c a n be

damage.

set u p s e p a r a t e g r o u n d i n g

various electrical parts

currents

but the i n d u c e d v o l t a g e s

level

short

short w i l l continue until

A constant voltage unit

the l i n e v o l t a g e b e l o w i t s s t e a d y s t a t e v a l u e ,

the c r o w b a r d e v i c e o f t e n w i l l .

there is a continuing follow

T h i s c o u l d be a p r o b l e m i f

current.

Constant voltage devices in everyday use are zener diodes and v a r i s t o r s ,

avalanche and

or voltage dependent

S p a r k gaps and gas d i s c h a r g e tubes a r e

resistors.

the m o s t c o m m o n type of

crowbar. Low

pass filters are

capacitor filter,

often used as

suppression devices.

p l a c e d a c r o s s the t e r m i n a l s i s the s i m p l e s t f o r m

w h e r e t h e i m p e d e n c e i t s h o u l d p r e s e n t to t h e

w i l l be m u c h l o w e r t h a n the t r a n s i e n t

A of

transient

source

impedence.

a p p r o a c h w i l l w o r k w e l l u n l e s s the c a p a c i t o r

loads down

This the

d e s i r e d voltage and does not c r e a t e c u r r e n t i n - r u s h p r o b l e m s . A

resistor

in series

of t h e f i l t e r .

will help,

A capacitor

but w i l l r e d u c e

transient has high energy i n either p o l a r i t y . come

the

effectiveness

n e t w o r k i s a l s o i n e f f e c t i v e i f the F i l t e r s can

be-

e x p e n s i v e a n d m u s t be v e r y c a r e f u l l y d e s i g n e d .

Isolation t r a n s f o r m e r s

m a y allow surges

c o u p l e d a c r o s s the w i n d i n g s , system.

to b e c a p a c i t i v e l y

t h e r e b y p a s s i n g t h e m i n t o the

A t t i m e s the l o a d m a y be s u c h that the i n p u t s u r g e

d i f f e r e n t i a t e d at the i s o l a t i o n t r a n s f o r m e r , rapid r i s e t i m e and hence,

causing a much

m a k i n g it m o r e d a n g e r o u s .

Con-

s i d e r a b l e thought a n d c a l c u l a t i o n m u s t be p e r f o r m e d b e f o r e c i d i n g to p r o t e c t The

by such a

de-

transformer.

l e a d l e n g t h of s u p p r e s s i o n d e v i c e s c a n c a u s e

overshoot voltages

is more

d e p e n d i n g on the r a t e of r i s e

of the

large current.


4.

BENT

It w a s

Lightning

and the

discussed earlier

overshoot

of o v e r

chase most

1300

Hazards

It

that a one i n c h w i r e c a n l e a d to a

volts.

It i s p o s s i b l e ,

protection devices

voltage

components most

must

equipment

be k n o w n o r levels.

c o m m o n types

the

circuit

estimated.

standards

Therefore,

m u s t be o b t a i n e d ,

f o r m e d or conservative

and

and devices,

conservatively

and c i r c u i t components,

from manufacturers

voltage

to p u r -

w i l l be n e g l i g i b l e .

w i t h s t a n d l e v e l of e q u i p m e n t s a n d

for lightning transient


overshoot

s e l e c t i o n of p r o t e c t i o n c o m p o n e n t s

transient

however,

in d i s c f o r m without w i r e s ,

w i t h c a r e f u l m o u n t i n g the v o l t a g e For

113

Produces

For

do not

exist

information available

laboratory

testing

engineering estimates used.

of e q u i p m e n t s a n d c o m p o n e n t s

are

per-

Limits for

provided for

guidance. Transistors

and Integrated C i r c u i t s :

2 times

normal

times

voltage.

voltage. Diodes:

1.5

Small motors, 10 t i m e s

their

n o r m a l operating

Large motors, 20 t i m e s

large

In a d d i t i o n t o t h e a b o v e ,

voltage As power

is

and light m a c h i n e r y :

voltage.

transformers

n o r m a l operating

and heavy

are

many times

punch-through voltage

limiting transients

to

1. 5 t i m e s

overlooked

for

transients

the D C w o r k i n g

recommended.

a n i n d i c a t i o n of l i n e t r a n s i e n t / s u r g e s , l i n e of 480

monitored for

v o l t s to a s a t e l l i t e

a

tracking

commercial

station

was

s p i k e s g r e a t e r than one j o u l e b y O d e n b u r g

T h e n u m b e r of s u r g e s

recorded

i n t e r e s t i n g to n o t e that the have

is d i s p l a y e d in F i g u r e

53, 020

alone is in keeping with other transients

machinery:

voltage.

capacitors

and unless their d i e l e c t r i c is known,

peak inverse

small transformers

been r e c o r d e d

surges

monitored in

published data.

It

is

September

At times

i n a 24 h o u r p e r i o d ,

(11).

20. 10,

000

m a n y of t h e m

occurring within m i l l i s e c o n d periods. A v a l a n c h e Diodes and Z e n e r s . istics

of a s e m i c o n d u c t o r

there are

diode are

three principle regions

The volt-ampere

of o p e r a t i o n .

b i a s e d r e g i o n i s l i m i t e d b y the e x t e r n a l region is where

the v o l t a g e

is r e v e r s e d ,

circuit,

W h e n the r e v e r s e v o l t a g e

this c r i t i c a l v a l u e ,

the r e v e r s e c u r r e n t

diodes are region,

made

increases

i n the b r e a k d o w n r e g i o n . suppressor

a n d the

increases

to o p e r a t e i n the f o r w a r d a n d

but t r a n s i e n t

The forward leakage

but it is s t i l l l e s s

the c r i t i c a l v a l u e .

the d i o d e i s o p e r a t i n g

character-

shown in F i g u r e 2 1 in which

than

beyond

s h a r p l y and

Normal

rectifier

reverse-biased

o p e r a t e a r o u n d the

breakdown

region.


114



at times over 10,000 surges in a 24 hour period

30k

25k

20k

1

Feb 1976

Figure 20.

M a r A p r M a y Jun July Aug Sept

Oct Nov Dec Jan !977

Surges greater than one joule suppressed at a USAF site, Odenburg


4.

BENT

Lightning

and

the

Hazards

It

115

Produces

A v a l a n c h e d i o d e s e x h i b i t a s h a r p t u r n at the k n e e , d i o d e s go t h r o u g h t h i s t r a n s i t i o n m o r e that the a v a l a n c h e than z e n e r

diode is a better

These devices

are

the m o s t

"constant

where

the e n e r g y


silicon avalanche

a junction area diode.

over

use

larger

size,

to s e v e r a l h u n d r e d k W f o r a capacitance,

however,

devices

small volume

b y the

transient

Special suppression

w i l l c l a m p at

d e p e n d i n g on the

within a v e r y

technique are

ten t i m e s

These devices

and,

transients

voltage"

or heat generated

c a n c a u s e f a i l u r e at the j u n c t i o n . u s i n g the

for

zener

implies

i s o n l y s l i g h t l y d e p e n d e n t o n the

T h e operation takes place

of s i l i c o n ,

sec

suppressor

but

This

diodes.

a v a i l a b l e a n d the v o l t a g e current.

gradually.

that

have

than a one watt z e n e r speeds

i n e x c e s s of

the p e a k p o w e r

1 |d s e c

devices

manufactured

pulse.

^

10

r a t i n g c a n be u p

T h e y do have a

small

but w i t h c a r e f u l d e s i g n it i s p o s s i b l e

them in protection circuits

at f r e q u e n c i e s

in excess

to

of

100 M H z .

V a r i s t o r s - V o l t a g e Dependent Resistors. bulk semiconductor

device whose

A varistor

resistance varies

m a g n i t u d e but n o t the p o l a r i t y of the a p p l i e d v o l t a g e . are

composed

and

heating

of a p o l y c r y s t a l l i n e m a t e r i a l m a d e

special mixtures

(SiC) or oxides

containing either

of z i n c a n d b i s m u t h .

( M O V ' s ) have a m o r e better c l a m p i n g . surges.

silicon carbide

Metal-oxide

devices

of m a g n i t u d e , a

therefore developed

i n the

resistance d i m i n i s h e s by s e v e r a l

t h u s a b s o r b i n g the e n e r g y

voltage

the M O V p r e s e n t s

a v e r y h i g h r e s i s t a n c e at i t s t e r m i n a l s ; h o w e v e r , its

varistors

f r o m induced

In the a b s e n c e o f a b n o r m a l v o l t a g e s ,

s e n c e of a s u r g e ,

Varistors

highly nonlinear elements

r e c e n t l y f o r p r o t e c t i o n of e l e c t r i c

a

by p r e s s i n g

nonlinear V - I relationship and

They are

is

w i t h the

preorders

of the t r a n s i e n t

above

specified value. MOV's

current

provide low voltage

characteristics worse

nonlinear elements than z e n e r

diodes,

with

b i - p o l a r p r o p e r t y and high energy d i s s i p a t i o n / s i z e These devices, power

lines,

p r i m a r i l y intended for

step response

Typical V - I curves Gas edness

are

the

O n the

available.

50 n a n o s e c o n d


Breakdown Devices.

signal line

of M O V b e c o m e

of a n M O V i s i n the

spectrum are

(GDT's).

types

capability.

p r o t e c t i o n of A C

w i l l be a l s o a p p l i c a b l e to l o w v o l t a g e

protection when lower voltage The

surge

voltage-

but w i t h a

region.

22.

o p p o s i t e e n d of the

s p a r k gaps and g a s - d i s c h a r g e

T h e s e d e p e n d o n the f o r m a t i o n of a n i o n o i z e d


ruggtubes gas



116

Figure 21.

10

FORWARD

BIAS

REVERSE

BIAS

+

Schematic volt-ampere characteristic curve for a semiconductor diode

10

10 "

10*

10

C u r r e n t (A) Figure 22.

Volt-intensity curve for MOV


4.

BENT

Lightning

and the Hazards

between m e t a l electrodes. s e v e r a l other f a c t o r s an arc

is formed,

currents

T h e gap length,

(~


are

state p o w e r

source

reaches

response

a high voltage

Druyvesteyn and Penning tant g l o w a n d a r c

is frequently

Many

time,

discharge

w h e r e one c a n see The arc

discharge

of a G D T a r e

(14),

here.

Its u s e i n t h e

s u r g e is shown i n F i g u r e 25,

r e g i o n c a n be s u s t a i n e d a t a l o w v o l t a g e ,

but

d e p e n d i n g o n the p o w e r

f i c a n t a n d m a y be s u f f i c i e n t to c a u s e As

the v o l t a g e p a s s e s

because

gas

ionized,

This

m a y be

to the

if the e l e c t r o d e s

are

It i s o f t e n a n a d v a n t a g e

p r o t e c t i o n to c l a m p t h e i n i t i a l v o l t a g e not c a p a b l e of p r o t e c t i n g a g a i n s t .

to p r o v i d e a d d e d

o v e r s h o o t that the G D T i s

This

c a n be d o n e i n

b y d e s i g n i n g h y b r i d c i r c u i t s w i t h the g a s

current protector initial

The

current

b u t c a n n o t be u s e d e f f e c t i v e l y i n p r o t e c t i n g l o w i n p u t

impedance circuits.

ways

hot

it m a y r e - i g n i t e o n the n e x t h a l f c y c l e .

tube is a n e x c e l l e n t d e v i c e f o r p r o t e c t i n g a g a i n s t h i g h

surges,

signi-

electrodes.

t h r o u g h z e r o at the e n d of e v e r y h a l f c y c l e ,

the G D T w i l l e x t i n g u i s h but at t i m e s , a n d the g a s

source,

damage

24

pro-

the A C v o l t a g e

m a y be s u f f i c i e n t to a l l o w a f o l l o w o r h o l d o v e r c u r r e n t . holdover current,

use

and


indicating an initial high c l a m p i n g voltage. t e c t i o n of a n A C l i n e

passing

A n e x c e l l e n t d e s c r i p t i o n of the

t h e i r m a i n c o n c l u s i o n s w i l l be b r i e f l y d e s c r i b e d

the a r c

gas

the i m p o r -

of s p a r k g a p s i s g i v e n i n a r e p o r t b y H a r t a n d H i g g i n s Typical volt-time curves

can

volts.

(1_3) d e s c r i b e t h e a c t i o n o f a

regions.

at l o w v o l t a g e .

such

s u c h that a

b e f o r e the a r c

n o t g e n e r a l l y f e a s i b l e b e l o w 90

d i s c h a r g e d e v i c e i n F i g u r e 23, high currents

and When

conducting until current and voltage

t e m p o r a r i l y d i s a b l i n g the s u p p l y .

fast-rising transient GDT's

pressure,

100V).

the s t e a d y

s u p p r e s s o r s also have a noticeable form.

gas

the s u p p r e s s o r i s c a p a b l e of c o n d u c t i n g h i g h

c a p a b l e of k e e p i n g the a r c reduced,

117

Produces

d e t e r m i n e the b r e a k d o w n v o l t a g e .

at a l o w v o l t a g e

Unfortunately, are

It

tube as

a n d a s o l i d - s t a t e d e v i c e to p r o t e c t

several the

initial

against

the

overshoot.

Results from

Protected

Systems

D u r i n g the l a s t two y e a r s

Atlantic Scientific Corporation

have been working with a large factures

O h i o b a s e d c o m p a n y that m a n u -

brain and body scanners

commonly called C A T scanners graphy),

and e a c h c o n s i s t s

microprocessor It w a s

for hospitals.

These

(Computerized Axial

of a l a r g e

are

Tomo-

computer and several

other

systems.

c o m m o n for some

systems

to h a v e o c c a s i o n a l b a d l y




118

10"

w*

w

6

w TIME

Figure 24.

s

70

10*

(SECONDS)

Small spark gap characteristics, Joslyn, 2301-14.


10

BENT

Lightning

and the Hazards

It Produces


4.

Figure 25.

Volt-time curves of transient with and without spark gap protection


119

120


formed images

o r f o r the c o m p u t e r

s t r u c t i o n s d u r i n g the c o u r s e vided with solid-state

to s u d d e n l y i s s u e f a l s e i n -

of a s c a n .

S e v e r a l units were

surge and transient

suppression.

pro-

Those

r a n g e d f r o m f r o n t e n d d e v i c e s c a p a b l e of c o n t r o l l i n g s u r g e s 2 5 , 000

A,

to l o w v o l t a g e c i r c u i t p r o t e c t o r s

in picosecond times Surprising time,

sec).

results were

system damage

Denver,


(10"^

obtained in terms

costs and s c a n q u a l i t y .

of s y s t e m

down-

One hospital in

C o l o r a d o i n d i c a t e d t h a t m a n y of t h e i r s o f t w a r e

dissappeared.

of

c a p a b l e of c l a m p i n g

problems

T h e s e w e r e f a l s e i n d i c a t i o n s that a s c a n had been

c o m p l e t e d and a l s o o c c a s i o n a l automatic patient table

movement

when not a s k e d f o r .

the i m -

A more

r e m a r k a b l e feature was

proved performance in s y s t e m downtime and electrical savings.

The graphs

t i m e f o r two F l o r i d a since kit installation. one of t h e s e s i t e s w a s over

190

tection.

systems

s h o w n i n F i g u r e 26 systems,

cost

s h o w the p e r c e n t

The P. C .

b o a r d r e p a i r c o s t s a v i n g s on

o v e r $ 4 , 000

per month.

S i n c e that t i m e ,

have been retrofit with solid-state

surge

built c o n n e c t o r / p r o t e c t o r Unfortunately, and p o w e r

pre-

c o m b i n a t i o n to b e i n c o r p o r a t e d .

the p r o t e c t o r

provided in many O E M c o m -

supplies is for overvoltage or

overcurrent

p r o t e c t i o n f o r f a i l i n g components, and its r e s p o n s e too l o n g f o r the r a p i d l y r i s i n g the

pro-

M a n y o f the u n i t s w e r e d e s i g n e d f o r e a s e o f i n s t a l l a t i o n

a n d n e e d e d o n l y s e p a r a t i o n o f a p l u g a n d s o c k e t to a l l o w a

puters

down-

with virtually zero downtime

surges

time is

and transients

far

entering

system. T h e r e s u l t s f r o m t h i s s t u d y o n the C A T s c a n n e r s

that s y s t e m s

c a n be a d e q u a t e l y p r o t e c t e d ,

indicates

but c a u t i o n m u s t

p l a c e d on the p e r f o r m a n c e of a n y o f f - t h e - s h e l f b l a c k b o x . k n o w l e d g e of its c o n t e n t s

be A

i s a m u s t i n o r d e r to a l l o w c o r r e c t

d e v i c e s e l e c t i o n a n d p r o v i d e the a d e q u a t e p r o t e c t i o n r e q u i r e d .


4.

BENT

Lightning

and

the

Hazards

It

Lightning W a r n i n g and T r a c k i n g Most instruments

and techniques

for

lightning warning,

r a n g e or

o c c u r r i n g and an excellent

comprehensive

One The

l o c a t i o n at w h i c h l i g h t n i n g i s

justifiably acceptable approaches

f i r s t d e s i g n of a w a r n i n g d e v i c e b u i l t i n 1962;

until

c o m b i n i n g t h e s e two

this e m p l o y e d a corona

i s n e c e s s a r y as

tech-

p o i n t to

l i g h t n i n g w a r n i n g that a r e i s t i n g l i g h t n i n g , a n d the

important.

measure

i t s e l f to g r o u n d f o r the f i r s t

c h a r g e b u i l d u p i n a c l o u d i s to u s e

of

of

of a c h a r g e d

m o d e r n and satisfactory method

e l e c t r i c field below it.

The

two a s p e c t s

O n e i s the d e t e c t i o n

o t h e r i s the d e t e c t i o n

t h a t i s a b o u t to d i s c h a r g e most

there are

re-

recordings.

a n d a c i r c u i t to d e t e c t e l e c t r i c f i e l d c h a n g e s .

combination approach

The

the

(8).

b e e n a c o m b i n a t i o n of f i e l d a n d f i e l d c h a n g e

niques was the f i e l d ,

d e s c r i p t i o n of

given by C i a n o s and P i e r c e

of t h e m o r e

cently has


Instrumentation

f u n c t i o n b y m o n i t o r i n g the m a j o r i t y of t h e m i s

121

Produces

time.

of m o n i t o r i n g

a f i e l d m i l l to m e a s u r e

When lightning develops

i n the

is affected Field

by wind Mill.

cloud,

current

to

indication

speed.

Fair

weather

a t a l o w p o s i t i v e v a l u e o f + 100 commonly produce

A n y corona

electric fields are V / m ,

and s t o r m

h i g h f i e l d s i n the r a n g e

a s t o r m is m o v i n g in,the v e r y

of - 5, 000

sudden excursions

is building up in a cloud overhead,

typically

measured

conditions

to c l o u d d i s c h a r g i n g c a n b e e a s i l y r e c o g n i z e d .

V / m .

When

i n the f i e l d W h e n the

due

charge

a c h a n g e i n the p o l a r i t y of

e l e c t r i c f i e l d c a n b e o b s e r v e d f r o m the p o s i t i v e f a i r

weather

value

a field

- 2 , 000

to a h i g h n e g a t i v e

value.

V / m c a n be u s e d as

is possible.

However,

For

a first

such

accurate information.

peculiarities make

of s t o r m

development

a p e r i o d of t i m e

There are

can

problems

give

with

f i e l d s and c o n c e a l i n g of true f i e l d s

that

a fixed value warning unreliable.

Considering negative

overhead

and m o r e

distant

c h a r g e c e n t e r i n the l o w e r p a r t

relative

clouds,

the

distances

and angular

o f the c l o u d .

relationships

field

strong

though,

Depending the

p o s i t i v e c h a r g e c e n t e r at the v e r y b a s e of the c l o u d c o u l d times

of

s h o u l d not be r e l i e d o n

s t u d y i n g the

m i l l r e a d i n g r e s p o n d s m o s t l y to t h e f i e l d s d u e to t h e on

the

e s t i m a t e that l i g h t n i n g

such a fixed value

b u i l d u p of the e l e c t r i c f i e l d o v e r

much more

situations,

order

solely for determining warning levels; and

the

the

t h e r e i s a n e l e c t r o s t a t i c f i e l d c h a n g e that c a n be m o n i t o r e d g i v e a n e s t i m a t e of f l a s h d i s t a n c e .

ex-

cloud

have a dominating effect,

c h a r g e i n the u p p e r p a r t

a n d s o c o u l d the

of a m o r e

distant

cloud.

large Such

small some-

positive variable


122

TOXIC C H E M I C A L A N D EXPLOSIVES FACILITIES

Melbourne, Florida

aver. per month

$39,000

$3,714 $

0

$65,000

$4,062

$

0

costs since kit

kit installed

•

40 -

costs before kit

20 -


o

40.

• Nov

V i " i — May Jan Mar

:\

J

r 0

40-

1 Mar

1 Sept

July

1 Nov

i Jan

Mar

'!

Temple University

20.

^\

\

kh Ml

J

f May

/

kit installed

\

— r — Jul y Sept Nov

Jan

i Mar

St. Petersburg, Florida kit installed

20. 0

^

\

—i Aug Figure 26.

Figure 27.

/

y

1 Oct

, 1 Dec 1

,

,

i

\ i

,

i Feb Apr Jun Aug

Advantages of surge protection on large system

Electrostatic field change from lightning as a function of flash distance


4.

BENT

Lightning

and the Hazards

c o n d i t i o n s h a v e to

It

Produces

be r e c o g n i z e d i n o r d e r

i n t e r p r e t a t i o n s of the f i e l d m i l l

123

to a c h i e v e

The major problem in determining correct caused by space

charge

screening.

by c o r o n a g i v e n off f r o m vegetation sources fumes,

etc.

sharp points on structures


r e l i a b l e d e t e r m i n a t i o n of c h a r g e

round structure

s u c h as a water

Field Change Equipment. i n the e l e c t r i c the r e c e i v e r

tower.

build up,

T h e f i e l d at of

T h e i n s t r u m e n t counts

f i e l d c a u s e d b y l i g h t n i n g to the d i s t a n c e

a n t e n n a a n d the l i g h t n i n g f l a s h .

3 V / m a t 4 0 m i l e s a n d 5, 0 0 0 o c c u r at a g r e a t d i s t a n c e ,

somewhat closer selectiveness

change

between

It a l s o t e n d s t o

Flashes

current

t h e n a f a l s e i n d i c a t i o n of a Considering however

of the e q u i p m e n t a n d the p h y s i c s of the

it i s s h o w n b y the f o l l o w i n g a r g u m e n t s

t h a t the u n c e r t a i n t y i n the

great.

i n s t r u m e n t s h o u l d be d e s i g n e d to o p e r a t e 0 t o 100 m i l e s .

i n the

fre-

T h i s d e t e r m i n e s that the

m e n t r e s p o n d s e s s e n t i a l l y to the n e a r f i e l d e l e c t r o s t a t i c p o n e n t of the e l e c t r i c

field change,

the d i s t a n c e d e t e r m i n a t i o n i s q u i t e r e l i a b l e .

3

and hence,

The 1/d

relation-

3

s h i p i s b a s e d o n the a p p r o x i m a t i o n that t h e l e n g t h of the c h a n n e l i s s m a l l c o m p a r e d w i t h i t s d i s t a n c e f r o m the Within a

deviations f r o m

10km range,

Figure 27illustrates

the

com-

The radiation component

of the e l e c t r i c f i e l d c h a n g e v a r i e s w i t h ( 1 / d i s t a n c e )

hence,

over

equip-

which for these conditions is

d o m i n a n t o v e r the r a d i a t i o n c o m p o n e n t .

point.

the

problem,

q u e n c y r a n g e b e l o w 1 k H z , a n d t h u s be s e n s i t i v e to l i g h t n i n g a distance f r o m

dis-

T h i s f i e l d change i s on

V / m at 3 m i l e s .

s t r o k e w i l l be g i v e n .

d i s t a n c e d e t e r m i n a t i o n is not so

such a

lightning

not of i d e n t i c a l i n t e n s i t y , a n d i f s o m e v e r y l a r g e

The

a

3.

e s s e n t i a l l y b y r e l a t i n g the s i z e of the

c r i m i n a t e i n f a v o r of g r o u n d d i s c h a r g e s .

strokes

mask

and produce false field indications.

s h o u l d b e p l a c e d a b o v e m u c h o f the c o r o n a o n t o p o f a

flashes and operates

are

and natural

also by ions in exhaust

l o c a t i o n w o u l d o n l y be e n h a n c e d b y a b o u t a f a c t o r

average,

is

S u c h c h a r g e d r e g i o n s c l o s e to a f i e l d m i l l c a n

T o obtain a m o r e large

field values

Ion c l o u d s c a n be f o r m e d

under high fields,

the e f f e c t of the c l o u d o v e r h e a d , field m i l l

accurate

data.

lightning

observation

this does not a l w a y s hold true and

1/d

3

r e l a t i o n c a n be

some electrostatic

m e n t s w h i c h f o l l o w the t h e o r e t i c a l c u r v e U s i n g t h i s t y p e of e q u i p m e n t a s

expected.

field change

measure-

closely.

the b a s i c

sensor,

Atlantic

S c i e n t i f i c C o r p o r a t i o n d e v e l o p e d a n e w i n s t r u m e n t that

integrates

the n u m b e r of l i g h t n i n g c o u n t s o v e r a v a r i a b l e t i m e i n t e r v a l f r o m 10 t o 6 0 counts

seconds and presents

a d i g i t a l d i s p l a y of the n u m b e r of

separated in four distance

ranges,

0-5

miles,

5-10


miles,


124

Electric Field (V/m) 105786


n

— c

21

S I o * ; ; u&b

• ; ;

16**5

1

;

,\i*tsx

tUal;TJimle!(h*$)! j | < • fiif-- -i- '

v ct

d v.

I -t— • T • -

mm

1

^IK! *: :fti :

:

TT

| ...j:

I I If

1

1

1 1

|;;.jvl::;j!k:LL:L:fel: "11 r

Figure 28. Results for lightning warning instrumentation at a naval station. The top three graphs are of electric field and the lower three of electric field change excursions greater than three preset values.



4.

BENT

Lightning and the Hazards It Produces

125

10-20 miles and 20-100 miles. An integration time of one minute makes erroneous distance readings easier to recognize, since the integrated count at the incorrect distance is much smaller than the simultaneous count in the correct distance range. An audible warning that can be set at any range level is also included. In addition, an output of each lightning strike separated into the four distance ranges was provided for the recorder. Figure 28 illustrates some results of this equipment showing electric field charge build up and warning several minutes prior to close lightning. Lightning Position and Tracking. An extremely sophisticated Lightning Position and Tracking System (LPATS) has been designed to locate and track thunderstorms out to 300 or 400 miles with an accuracy of better than 3 degrees. Lightning location techniques by triangulation beyond 15 0 km have been in existance around the globe since World War II which provide good accuracy, but for close lightning the tortuous channel as well as ionospheric reflections lead to large errors. LPATS relies on being able to detect the cloud to ground discharge by its unique broadband magnetic field waveform. Once detected, this waveform is sampled for the part of the return stroke that is within 100 feet of the ground. It is well known that this part of the discharge is almost always vertical and carries the greatest energy, implying that we have a vertical amni-directional radiating antenna and a powerful transmitter The system monitors ground stroke location, storm center and speed and direction of movement, as well as storm intensity and its variations. It can resolve multiple storms and is capable of displaying the information in map form on a TV screen. Additional advantages of the system are its capability of monitoring "hot" lightning which is known to start forest fires, and its ability to investigate its own performance and malfunctionso 0

ABSTRACT A general understanding of the basic lightning process can lead to a much better understanding of lightning protection techniques and the resulting level of protection. The design of satisfactory lightning protection systems can only be achieved with a thorough knowledge of the mechanism and characteristics of a lightning strike, and the related problems that a steep voltage wavefront has on inadequate bonding and grounding.



126


Lightning induced line surges can also cause major damage to electrical or electronic systems such as computers, and it may also change data or programs without any permanent damage. The resulting effects can be disastrous where chemical mixing is governed by electronic techniques. A considerable portion of the damage caused by such transients and surges can be eliminated with careful planning of protection equipment, but the advent of solid-state components has placed considerable emphasis on the term "careful planning". This paper discusses a l l these points and also attempts to educate the reader in lightning protection and statistics as well as lightning warning systems. References 1.

Llewellyn, S. K., Broadband magnetic waveforms radiated from lightning, M.S. Thesis, Florida Institute of Technology, Melbourne, F l o r i d a , 1977.

2.

Anderson, J. G. and K . O. Tangen, Insulation of switchingsurge voltages, in E H V Transmission Line Reference Book, Edison E l e c t r i c Inst., New York, 1968.

3.

Golde, R. H., Lightning protection, Edward Arnold Ltd, London, 1973.

4.

Office of Naval Research, Code 450, Review of lightning protection technology for tall structures, Conference proceedings, 1975.

5.

Bent, R. B.and S. K . Llewellyn, A n investigation of the lightning elimination and strike reduction properties of dissipation a r r a y s , Report No. FAA-RD-77-19, 1976.

6.

Smith, R. S., Lightning protection for facilities housing electronic equipment, F A A - R D - 7 7 - 8 4 , May, 1977.

7.

H i l l , R. D., Thunderbolts, Endeavour, 31, No. 112, 1972.

8.

Cianos, N . and E. T. Pierce, Methods for lightning warning and avoidance, SRI Tech. Report 1, 1974.

9.

Pierce, E . T . and P r i c e , G . H., Natural electrical effects on the operation of tethered balloon systems. Stanford Research Institute Project 3058, M a r c h , 1974.


3-9,

4.

BENT

Lightning

and the Hazards

It

Produces

127

10. Horvath, T., Gleichwertige Fläche und relative Einschlagsgefahr als charakteristische Ausdrücke des Schutzeffektes von Blitzableitern. Int. Blitzschutzkonferenz, Munich, 1971.


11. Odenberg, R., Protecting facilities from induced lightning and power line switching transients, F A A - R D - 7 7 - 8 4 , May, 1977. 12. F i s h e r , F . A., Instruction Bulletin 53E 9007, Fischer and Porter Co., Warminster, Penn., 1970. 13. Druyvesteyn, M. J. and F. M. Penning, E l e c t r i c a l discharges in gases , Rev. Mod Physics, 12, p. 87, 1940. a

14. Hart, W. C., and D . F . Higgins, A guide to the use of spark gaps for electromagnetic pulse (EMP) protection, Report No. JES-198-1M-11/75, Joslyn Electronic Systems, Goleta, Ca., 1972. RECEIVED November 22,

1978.


Lightning and the Hazards It Produces for Explosive Facilities - ACS

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